1. Field of the Invention
The present invention relates to a radiation imaging system using radiation to capture an image of a subject and an image processing method for a radiation imaging system, and particularly to a radiation imaging system using a fringe scanning method and an image processing method for a radiation imaging system using a fringe scanning method.
2. Description Related to the Prior Art
Radiation, for example, X-rays, is attenuated depending on an atomic number of an element constituting a substance, and density and thickness of a substance. By taking advantage of such properties, the X-rays are used as a probe for examining the inside of a subject in medical diagnoses and non-destructive inspections.
A common X-ray imaging system captures a transmission image of a subject disposed between an X-ray source that emits the X-rays and an X-ray image detector that detects the X-rays. The X-rays, emitted from the X-ray source toward the X-ray image detector, are attenuated (absorbed) by a substance, disposed on a path toward the X-ray image detector, by an amount corresponding to differences in properties (the atomic number, the density, and the thickness) of the elements in the substance. Then the X-rays are incident on each pixel of the X-ray image detector. Thus, the X-ray image detector detects and images an X-ray absorption image of the subject. Stimulable phosphor panels and flat panel detectors (FPDs) using semiconductor circuits are widely used as the X-ray image detectors.
The X-ray absorption performance of the substance decreases as the atomic number of the element constituting the substance decreases. This causes a problem that sufficient contrast cannot be obtained in the X-ray absorption image of living soft tissue or soft materials. For example, a cartilage portion constituting a joint of a human body and synovial fluid surrounding the cartilage portion are mainly composed of water, so that there is little difference between their amounts of X-ray absorption, resulting in little difference in contrast.
Due to such background, X-ray phase imaging has been actively studied recently. The X-ray phase imaging is used to obtain an image (hereafter referred to as the phase contrast image) based on phase shifts (angular changes), instead of intensity changes, of the X-rays caused by the subject. Generally, when the X-rays are incident on the subject, the subject interacts with the phase of the X-rays more strongly than with the intensity of the X-rays. Thus, the X-ray phase imaging using phase difference provides a high contrast image even if the subject has low X-ray absorption properties. An X-ray imaging system using an X-ray Talbot interferometer is known as one type of the X-ray phase imaging. The X-ray Talbot interferometer is composed of two transmission-type diffraction gratings and an X-ray image detector (see, for example, U.S. Pat. No. 7,180,979 corresponding to Japanese Patent No. 4445397 and C. David et al., Applied Physics Letters, Vol. 81, No. 17, October 2002, page 3287)
In the X-ray Talbot interferometer, the first diffraction grating is disposed behind the subject. The second diffraction grating is disposed downstream from the first diffraction grating by a Talbot length. The Talbot length is determined by a grating pitch of the first diffraction grating and an X-ray wavelength. The X-ray image detector is disposed behind the second diffraction grating. The Talbot length is a distance at which the X-rays passed through the first diffraction grating form a self-image (fringe image) due to Talbot effect. The self image is modulated by the phase shift of the X-rays caused by the subject disposed between the X-ray source and the first diffraction grating.
In the X-ray imaging system, the intensity of the fringe image is modulated by the superposition of the self image of the first diffraction grating onto the second diffraction grating. The phase contrast image of the subject is obtained from changes in the fringe image caused by the subject with the use of a fringe scanning method. In the fringe scanning method, the image is captured at each scan position with the second diffraction grating translationally moved (scanned) at a scanning pitch, being a fraction of the grating pitch, relative to the first diffraction grating in a direction substantially parallel with a plane of the first grating and substantially vertical to a direction of a grating line of the first diffraction grating. A phase shift differential image is produced from a phase shift value of an intensity modulation signal representing the intensity changes, relative to the respective scan positions, in pixel data of each pixel obtained with the X-ray image detector. The phase shift differential image corresponds to angular distribution of the X-rays refracted by the subject. The phase contrast image is produced by integrating the phase shift differential image in the direction of the scanning. The fringe scanning method is also employed in imaging apparatuses using laser (see, for example, Hector Canabal et al., Applied Optics, Vol. 37, No. 26, September 1998, page 6227).
In the fringe scanning method, the positional relationship between the first and second diffraction gratings strongly affects the image quality of the phase contrast image. Distortion, manufacturing error, arrangement error, or the like in the first or second diffraction grating results in offset, whose value corresponds to the distortion, the error, or the like, in the phase shift differential image. This deteriorates the image quality of the phase contrast image. In the U.S. Pat. No. 7,180,979, a phase shift differential image captured in the absence of a subject in preliminary imaging is stored as offset data. The offset data is subtracted from a phase shift differential image captured in the presence of the subject in main imaging. Thereby, the phase shift differential image produced reflects subject information only.
In the method for correcting the offset in the phase shift differential image disclosed in the U.S. Pat. No. 7,180,979, it is necessary that the preliminary imaging and the main imaging are performed under the same imaging conditions except for the presence and absence of the subject. When an initial position of the relative scanning of the first and second diffraction gratings in the main imaging is different from that in the preliminary imaging, artifact occurs due to the change in the initial position.
The artifact occurs due to an expression for calculating a phase shift value of an intensity modulation signal. As described in the U.S. Pat. No. 7,180,979, the phase shift value is calculated by extraction of argument in a complex plane, namely, an arctangent function (tan−1). The range is from −π/2 to π/2. As shown in FIG. 17A, when the phase shift differential image captured in the preliminary imaging contains moiré fringes due to the first and second diffraction gratings, a profile ψ1(x) with respect to the direction orthogonal to the moiré fringes is discontinuous across a portion (boundary) at which the value changes from −π/2 to +π/2 or from +π/2 to Hence, the profile ψ1(x) has a saw-like shape. The moiré fringes also appear in the phase shift differential image captured in the main imaging. As shown in FIG. 17B, a profile ψ2(x), in the direction orthogonal to the moiré fringes, has the saw-like shape in a similar manner.
When the initial position of the relative scanning of the first and second diffraction gratings in the main scanning is at the same position as that in the preliminary imaging, the profiles ψ1(x) and ψ2(x) have the same shape, so that they cancel out each other when subjected to the offset correction. When the initial position of the relative scanning in the main imaging is shifted from that in the preliminary imaging, there is a shift δ between the profiles ψ1(x) and ψ2(x). In this case, as shown in FIG. 17C, banding artifact with the value of approximately π appears in a subtraction image produced by the offset correction of the profiles ψ1(x) and ψ2(x).
The artifact appears not only when there is a change in the initial scanning position between the preliminary and main imaging, but also when there is a change in the positional relationship of the first and second diffraction gratings between the preliminary and main imaging.